Using retroviral vector expression systems, we have analyzed the role of ETS1, ETS2 and FLI1 on hematopoietic cell growth, differentiation, and transformation. ETS1-expressing viruses had been shown to induce increased erythroid character in both K562 and HEL cells, and ETS1 expression had been shown to increase following treatment with the erythroid inducers ARA-C and Hemin. Antisense ETS1 blocked the induction of increased levels of ETS1 protein by these inducers, and prevented the increase in benzidine staining, indicating that ETS1 plays a role. Our data also indicates the ETS-related transcription factor FLI1/ERG-B acts as a megakaryocytic-specific factor. TPA-treated human K562 cells express megakaryocytic markers and specifically express increased levels of FLI1, but not ETS1 or ETS2. Infection of K562 cells with a FLI1-expressing retroviral construct induces megakaryocytic-like phenotypic changes, as well as elevated expression of several megakaryocytic surface markers, including CD41a (gpIIb/IIIa) and CD49b. The level of FLI1 expression correlates directly with the degree of morphological alteration and the levels of megakaryocytic markers expressed, consistent with the hypothesis that FLI1 acts as a lineage-specific transcription factor in these cells.